This invention relates to a prosthetic limb utilizing an air cylinder device for assisting a bending and extending action of the knee, and more particularly to a technology capable of enlarging an effective stroke of an air cylinder device and effective in making a prosthetic limb small in size or in obtaining a motion in conformity with a walking speed of the user.
In general, artificial limbs accompanying with a bending and extending action of the knee comprises an upper member including a knee plate to which a load or weight of the user (amputee) applies, a lower member connected to the upper member at the knee portion, a knee axis rotatably connecting the upper and lower members together, and in addition, an auxiliary means for assisting a bending and extending action of the knee. One typical example of this type of prosthetic leg is an above-knee prosthesis which includes a socket supported on an upper end of the knee plate of the upper member and adapted to receive the stump therein, and a foot member supported on a lower end of the lower member.
A hydraulic cylinder device or an air cylinder device is used as the auxiliary means for assisting a bending and extending action of the knee. U.S. Pat. No. 2,859,451 discloses a prosthetic leg utilizing a hydraulic cylinder device, and U.S. Pat. No. 5,405,407 discloses a prosthetic leg utilizing an air cylinder device. The hydraulic cylinder device and the air cylinder device are different in fluid to be used but the same in function for assisting a bending and extending action of the knee, thereby giving a walking style resembling a natural walk to the prosthetic leg.
Comparing the hydraulic cylinder device with the air cylinder device both as an auxiliary means, an oil as a working fluid of the hydraulic cylinder device has almost no compressive properties. In contrast, an air as a working fluid of the air cylinder device has compressive properties. Therefore, according to the air cylinder device, a repulsive force can be obtained by a compressed energy generated by compressing the air after the knee is bent maximum. In order to effectively utilize such features of the air cylinder device, it is necessary to enlarge an effective stroke of the cylinder. However, any attempt to enlarge an effective stroke of the cylinder is encountered with a difficulty in which the attempt tends to enlarge the size of the prosthetic leg. For this reason, not many specific proposals were made until today for enlarging the effective stroke of the air cylinder device in this type of a prosthetic leg. For example, specific proposals are seen in the above-discussed US patents which teach that if a rod end side of the cylinder device is connected to the upper member through a pin, an available space around the knee will be increased. Also, Japanese Laid-Open Patent Application No. 551/1997 teaches an improvement of U.S. Pat. No. 5,405,407. What is illustrated in the drawings of this laid-open application is only such a very general knowledge that a connecting position of a pin on the rod end side is located backwardly of the knee axis. FIG. 2 of U.S. Pat. No. 2,859,451 and FIG. 4 of Japanese Laid-Open Patent Application No. 551/1997 show a connecting position of a pin on the rod end side which position is located backward and slightly upward of the knee axis. However, they merely show the connecting position of the pin in the drawings and none of them particularly specifies the connecting position. In addition, none of them teaches the technical significance of the connecting position of the pin.